1. Field of the Invention
The present invention relates generally to an apparatus and method for sensing an insertion state of a wafer in a wafer cassette, and more particularly, to an apparatus and method that optically detects a position of a wafer placed in the cassette and then determines whether or not the wafer is correctly inserted into the cassette slot using a timing device.
2. Description of the Related Art
During semiconductor device fabrication, wafers are generally loaded into cassettes for transfer within and between the various fabrication processes. The cassettes contain a plurality of adjacent slots into which the wafers are inserted. The slots may be oriented vertically, in which case the wafers are inserted horizontally into the cassette, or the slots may be oriented horizontally, in which case the wafers are inserted vertically into the cassette.
During a transfer operation, the wafers inserted into the cassette are carried to a handler of a specific piece of process equipment. A robot extracts each wafer from the cassette and places it in the process equipment, and then returns the wafer to the cassette after the process has been completed.
If the cassette slots are oriented vertically on a cassette stage, the robot loads/unloads the wafers in the cassette by the horizontal operation of a robot arm. Since the robot arm travels along a fixed path, the cassette stage must be vertically driven relative to the robot arm so that successive wafers can be removed by operation of the robot arm.
While the wafers are being inserted into or extracted from the cassette by the robot, optical sensing methods are employed to determine whether a wafer is present in a particular slot. Specifically, a light beam source and a, photo-detector are positioned at opposing sides of the cassette. If the beam passes through the slot and reaches the photo-detector, it is determined that a wafer is not present in the slot. If the beam does not reach the photo-detector, it is assumed that a wafer is blocking the beam and therefore it is determined that a wafer is in the slot. As a result, a wafer position of each slot is determined and the cassette stage is driven vertically to allow the robot to sequentially load/unload the wafers from the corresponding slot positions.
In this conventional method, the wafer position is derived from logic high or low signals that are output by the photo-detector, depending on whether or not the light beam reaches the photo-detector. However, if the wafer is incorrectly positioned in the slot, such as when it is obliquely inserted into the slot, or incompletely inserted into the slot, or if two wafers are inserted into one slot, the existence of the wafer is validated but the correct insertion state of the wafer is not checked or ascertained.
Such incorrectly positioned wafers may be broken by contact between the wafer and the robot arm during the wafer transfer. Also, contact between the robot and the carrier itself may cause the whole carrier to be shifted or vibrated, thereby damaging many of the wafers in the cassette.
Accordingly, a need exists for a system which senses the correct/incorrect insertion state of the wafer within a wafer cassette, and stops the loading/unloading operations if the wafer is incorrectly inserted.